The present invention relates to a rear shock-absorber unit consisting of a hydraulic damper of the expansion cylinder type, provided with spring action, which suspends the rear wheel of motorcycles, and particularly to an adjusting device which adjusts the damping force from outside of the piston of the said hydraulic damper.
The rear shock-absorber unit provide with spring action, which suspends the rear wheel of motorcycles generally keeps stays at the position of its extreme expansion the suspension spring acting on the unit, before it is mounted on the motorcycle body when it is detached from the body. It is preferable, therefore, to adjust the damping force when it is at the extreme of expansion rather than when it is in compression.
A well-known adjusting device of such a kind is described in the specification of U.S. Pat. No. 3,937,307, granted to Cornelis de Kock, dated Feb. 10, 1976. The damping force adjusting device described in the specification consists of a ring-shaped valve which has an orifice leading to the opening of the piston and which is fixed firmly to it, a closure member which changes the said orifice opening area by relative rotary displacement with respect to this valve, an adjusting member which integrally connects to the closure member, and a stop member which is fitted to the end member of the cylinder so as to move in-and out freely. When the piston is moved to the extreme of expansion under the condition of the said stop member protruding into the cylinder, the said adjusting member is engaged with this stop member, and during the time when the piston is rotated by the piston rod, the rotation of the said closure member is prevented by the engagement of the adjusting member and stop member, and the valve rotating with the piston produces a relative rotary displacement with respect to the closure member to change the orifice opening area, thus, the damping force is adjusted.
In this well-known device, however, since the damping force adjustment is practiced in a manner such that, with the relative rotary displacement between a ring-shaped valve fixed firmly to the piston and a closing member fitted rotationally on the valve, the opening area of an orifice formed in the said valve is controlled, the damping characteristics which are changed are not only those at the low-speed range of the piston which depend solely on the orifice but also those at the medium and high speed range of the piston which depend on the valve, and this causes the defect that the desired damping characteristics can not be produced throughout the whole range of the piston speed. There is also another defect that, during the expansion stroke when the piston goes up, no damping characteristics which depend on the valve may be obtained. It may be added that, among the defects, there is also the fact that a special stop member is required only for the purpose of adjusting the damping force.